Antiaromaticity and π-Electron Behavior of Cyclo[25]carbon Modulated by Odd-Topology Phase Frustration

Cyclo[ n ]carbons─molecular rings composed exclusively of sp-hybridized carbon atoms─represent a uniquely demanding frontier in carbon allotrope chemistry owing to their extreme structural sensitivity and unconventional π-electron behavior. The recent on-surface synthesis of cyclo[25] carbon (C 25 ), the largest odd-membered ring realized to date, provides an unprecedented opportunity to interrogate the fundamental properties of electron-deficient, symmetry-broken carbon nanostructures. Here we combine extensive first-principles calculations─spanning ground-state electronic structure, magnetic response, and thermodynamic stability─to elucidate the bonding and antiaromatic character of C 25 . We show that C 25 adopts a planar C s -symmetric triplet ground state featuring pronounced bond-length and bond-angle alternation, reflecting a delicate interplay between Peierls distortion and electron delocalization. π-Electron analysis reveals a striking departure from even-membered cyclocarbons: the π in subsystem supports simultaneous clockwise and counterclockwise induced ring currents under an external magnetic field. This counterintuitive duality originates from the intrinsic phase discontinuity of odd-membered rings, wherein the π in wave function cannot achieve periodic phase closure and undergoes a π phase inversion between the inner and outer regions of the ring. ACID, NICS and ICSS ZZ analyses collectively confirm that C 25 is globally antiaromatic, with its unusual current patterns arising from phase-frustrated π conjugation rather than conventional Hückel-type electron counting. NICS ZZ analysis of structures sampled from AIMD trajectories confirms the thermally robust weak antiaromaticity of C 25 without aromaticity inversion. Excited-state calculations further show that the dominant absorption features stem from strongly localized π → π* transitions involving mixed contributions from both π in and π out manifolds. These results identify C 25 as a phase-frustrated, open-shell cyclocarbon with structural and magnetic properties fundamentally distinct from those of even-membered homologues. The insights obtained here provide a basis for understanding antiaromaticity and excited-state behavior in odd-membered sp-carbon rings and may facilitate future studies of larger or functionalized cyclocarbons.